1. Field of the Invention
This invention relates to the electroless deposition of metal from non-autocatalytic electroless plating baths that are inherently self-limiting as to the thickness of metal which can be deposited. In particular, this invention relates to the electroless deposition of copper from non-autocatalytic electroless copper plating baths utilizing a reducing agent to reduce copper ions dissolved in the solution to metallic copper to provide metal deposits or films on a suitably prepared substrate contacted by the bath solution. An example of such a copper plating bath is disclosed in U.S. patent application Ser. No. 909,209, filed May 25, 1978, now U.S. Pat. No. 4,207,331 and assigned to the assignee of the present application.
In application Ser. No. 909,209, hypophosphite is disclosed as a reducing agent which is useful as a reducer for copper ions in electroless plating baths. The hypophosphite reduced copper system is inherently non-autocatalytic to copper, in contrast to presently known formaldehyde reduced systems which are inherently autocatalytic to copper, and thus results in copper depositions of limited thickness.
2. Description of the Prior Art
In U.S. patent application Ser. No. 964,128, filed Nov. 27, 1978, now U.S. Pat. No. 4,265,943 and assigned to the assignee of the present application, there is disclosed an invention which overcomes the self-limiting nature of a hypophosphite reduced and similar non-autocatalytic copper system so as to provide continuous plating, that is, plating to a desired thickness which increases with time at a substantially constant rate of metallic copper when utilizing a non-formaldehyde type reducing agent, electroless copper plating bath. Such is achieved, in the invention disclosed in application Ser. No. 964,128, by including, in the system, a source of non-copper ions, preferably nickel or cobalt ions. The plating obtained in that invention is said to be essentially that of copper with only small quantities of the non-copper metal being codeposited with copper. The codeposited metal is said to function as an autocatalysis promoter so that metallic copper deposits or films of a desired thickness, greater than the limiting thickness obtainable before, are obtained as a continuous plating step, that is, as a plating operation wherein the plating thickness increases with time at a substantially constant rate.
The discussion of the prior art in application Ser. Nos. 909,209 and 964,128, referred to above, both of which are incorporated by reference herein, reveals that it was surprising and unexpected that a non-formaldehyde type reducing agent such as hypophosphite, would successfully reduce copper ions to metallic copper for electroless deposition while yielding advantages not previously available in typical formaldehyde reduced systems and that the self-limiting nature of the hypophosphite reduced system could be overcome by the addition of nickel or cobalt ions so that the system was no longer non-autocatalytic.
Plating processes wherein electrolytic current is used in conjunction with electroless plating are known. For example, U.S. Pat. No. 2,644,787 discloses a combination of electroplating and chemical plating, in a bath containing nickel, cobalt and hypophosphite ions, onto a conductive substrate to produce a magnetic coating. The patent discloses current densities of 10 to 200 amps per sq. ft. in a bath solution having a pH from 2 to 6, and states that the process yields deposits in excess of those expected from the current flow.
U.S. Pat. No. 3,264,199 describes a process for depositing nickel coatings on a metallic object by an autocatalytic chemical reduction reaction in a bath containing nickel salt and hypophosphite. The uniformity of the rate of the electroless deposition process is said to be materially aided by the addition of superimposed direct current pulses at periodic intervals. The patent further states that the current provides an electrochemical action acting as an aid to the chemical reduction process and, in effect, increase the rate of deposition of the electroless process but that the addition of a small current density does not increase the rate due to an electroplating action.
In U.S. Pat. No. 3,303,111 there is described a process for depositing nickel on a metal surface from a nickel-hypophosphite bath using AC or DC current to speed up the deposition of nickel. The patent states that the process is not one of electroplating, but, rather, is one in which the applied current apparently supplements the galvanic potential of the hypophosphite and excites it to do work which it could not do without such supplemental energy.
U.S. Pat. No. 3,485,725 discloses controlling the rate of deposition of metal on to a non-metallic substrate from an electroless plating bath by impressing an electric potential on the surface of the substrate. An example in the patent shows the use of current in an autocatalytic nickel or cobalt-hypophosphite bath to increase or decrease plating rate. It is said that this results in a unique phenomenon in that more plate is achieved than expected from a yield based on the sum of the electroless and electrolytic action.
British Pat. No. 1,222,969 discloses a process of depositing a metal layer onto a substrate by an electroless plating process in an autocatalytic electroless plating bath followed by an electroplating process without removing the substrate from the plating solution.
The various patents discussed above disclose the use of an electric current in electroless copper, nickel or cobalt baths, all of which are autocatalytic. Thus, in the patents, the bath compositions disclosed invariably employ formaldehyde type reducing agents for copper formulations and hypophosphite type reducing agents for nickel formulations, similar to the disclosures made in the prior art discussed in application Ser. Nos. 909,209, and 964,128.
The electroless plating and electrolytic plating disclosed in the above patents continue concurrently with the electric current being employed primarily to boost the plating rate. The patents are silent with respect to electroless plating baths which are non-autocatalytic and inherently self-limiting as to plating thickness, such as the hypophosphite reduced bath disclosed in application Ser. No. 909,209. In contrast to formaldehyde reduced copper solutions and hypophosphite reduced nickel solutions, hypophosphite reduced copper solutions are not autocatalytic so that catalyzation ceases when the surface of the substrate is covered with a deposit at about 10 micro inches thick.
It is generally necessary to build up plate thicknesses greater than 10 micro inches for commercial uses. Even where conventional electroplating is to follow electroless plating to build plate thickness, such procedure is not commercially acceptable in many instances where the initial deposit obtained by electroless plating is too thin.
For example, when workpieces are removed from a non-autocatalytic electroless plating bath that has been self-limiting as to thickness so that a thin plating only has been obtained, there is the possibility that the connection between the parts or workpieces and the plating rack or fixtures will be broken, thus destroying the conductor bridge necessary to get electric current to the part. This increases the incidence of burnoff in subsequent electroplating.
Another possible disadvantage is the possibility of bipolar deplating due to the thin deposit. Bipolar deplating occurs where a low current density area of a workpiece becomes anodic, due to the limited current carrying capability of the thin deposit, to an adjacent area of the piece which is cathodically plating. This can cause dissolution of the thin anodic electroless deposit which, if too thin, can be completely stripped away.
In addition, workpieces that have been electrolessly plated with a thin deposit and stored prior to electroplating may require reactivation prior to plating by techniques which might remove copper which cannot be spared by thin deposits. Also, processes which require intermediate operation, such as imaging techniques in the manufacture printed circuits, often require greater copper thickness than can be provided in non-autocatalytic baths which are inherently self-limiting as to thickness.
While the method and composition disclosed in application Ser. No. 964,128 overcome the disadvantages discussed above, the plating obtained contains a minor amount of the noncopper ion, such as nickel or cobalt, added to the plating solution. In addition, the system provides for continuous plating, wherein the plating thickness increases with time at a substantially constant rate as a consequence of the deposit becoming autocatalytic, through a chemical change, that is, a change in the composition of the plating bath such as by the addition of nickel and cobalt. Thus, there exists a need for alternate means to overcome the disadvantages of non-autocatalytic electroless plating baths discussed above.